Zookeeper（58）如何在Zookeeper中实现分布式锁？

在 Zookeeper 中实现分布式锁是一种常见的用例。Zookeeper 提供了强一致性、高可用性的分布式协调服务，使得它非常适合用来实现分布式锁。以下是详细的步骤和代码示例，展示如何在 Zookeeper 中实现分布式锁。

1. Zookeeper 分布式锁的基本原理

Zookeeper 分布式锁的实现主要依赖于以下几点：

临时顺序节点：每个客户端在尝试获取锁时，会在特定节点下创建一个临时顺序节点。
节点排序：所有客户端创建的节点会按顺序排列，最小的节点获得锁。
监视前一个节点：每个客户端监视它前一个节点的变化，当前一个节点被删除时，当前客户端重新检查自己是否是最小的节点，如果是，则获得锁。

2. 添加依赖

首先，需要在项目中添加 Zookeeper 的依赖。以下是 Maven 和 Gradle 的依赖配置：

Maven

在 pom.xml 中添加 Zookeeper 依赖：

<dependency><groupId>org.apache.zookeeper</groupId><artifactId>zookeeper</artifactId><version>3.7.0</version>
</dependency>

Gradle

在 build.gradle 中添加 Zookeeper 依赖：

dependencies {implementation 'org.apache.zookeeper:zookeeper:3.7.0'
}

3. 实现分布式锁

以下是一个完整的实现分布式锁的示例代码。

ZookeeperClient.java

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;public class ZookeeperClient {private static final String ZK_ADDRESS = "localhost:2181";private static final int SESSION_TIMEOUT = 3000;private ZooKeeper zooKeeper;public void connect() throws Exception {zooKeeper = new ZooKeeper(ZK_ADDRESS, SESSION_TIMEOUT, new Watcher() {@Overridepublic void process(WatchedEvent event) {System.out.println("Received event: " + event);}});}public void close() throws InterruptedException {if (zooKeeper != null) {zooKeeper.close();}}public ZooKeeper getZooKeeper() {return zooKeeper;}public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();// 在这里可以测试分布式锁client.close();} catch (Exception e) {e.printStackTrace();}}
}

DistributedLock.java

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;import java.util.Collections;
import java.util.List;public class DistributedLock {private static final String LOCK_ROOT = "/locks";private static final String LOCK_NODE = LOCK_ROOT + "/lock_";private ZooKeeper zooKeeper;private String lockPath;public DistributedLock(ZooKeeper zooKeeper) throws Exception {this.zooKeeper = zooKeeper;Stat stat = zooKeeper.exists(LOCK_ROOT, false);if (stat == null) {zooKeeper.create(LOCK_ROOT, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);}}public void acquireLock() throws Exception {lockPath = zooKeeper.create(LOCK_NODE, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);System.out.println("Lock path: " + lockPath);while (true) {List<String> children = zooKeeper.getChildren(LOCK_ROOT, false);Collections.sort(children);String smallestChild = LOCK_ROOT + "/" + children.get(0);if (lockPath.equals(smallestChild)) {System.out.println("Acquired lock: " + lockPath);return;}String watchNode = null;for (int i = children.size() - 1; i >= 0; i--) {String child = LOCK_ROOT + "/" + children.get(i);if (child.compareTo(lockPath) < 0) {watchNode = child;break;}}if (watchNode != null) {final Object lock = new Object();Watcher watcher = new Watcher() {@Overridepublic void process(WatchedEvent event) {synchronized (lock) {lock.notifyAll();}}};Stat stat = zooKeeper.exists(watchNode, watcher);if (stat != null) {synchronized (lock) {lock.wait();}}}}}public void releaseLock() throws Exception {if (lockPath != null) {zooKeeper.delete(lockPath, -1);System.out.println("Released lock: " + lockPath);lockPath = null;}}public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();ZooKeeper zooKeeper = client.getZooKeeper();DistributedLock lock = new DistributedLock(zooKeeper);lock.acquireLock();// 模拟业务逻辑Thread.sleep(5000);lock.releaseLock();client.close();} catch (Exception e) {e.printStackTrace();}}
}

4. 代码说明

ZookeeperClient 类

ZookeeperClient 类负责连接和关闭 Zookeeper 客户端，并提供获取 ZooKeeper 实例的方法。

DistributedLock 类

DistributedLock 类负责实现分布式锁的逻辑，包括获取锁和释放锁。

acquireLock() 方法：尝试获取锁，创建临时顺序节点，然后检查自己是否是最小的节点。如果是，则获得锁；否则，监视前一个节点的变化。
releaseLock() 方法：释放锁，删除自己创建的临时顺序节点。

5. 测试分布式锁

在 DistributedLock 类的 main 方法中，创建 ZookeeperClient 实例并连接 Zookeeper，然后创建 DistributedLock 实例并尝试获取和释放锁。可以通过运行多个实例来测试分布式锁的功能。

public static void main(String[] args) {ZookeeperClient client = new ZookeeperClient();try {client.connect();ZooKeeper zooKeeper = client.getZooKeeper();DistributedLock lock = new DistributedLock(zooKeeper);lock.acquireLock();// 模拟业务逻辑Thread.sleep(5000);lock.releaseLock();client.close();} catch (Exception e) {e.printStackTrace();}
}